Ultrapure liquids are used in various industrial processes such as semiconductor wafer fabrication. Liquids such acetone, freon, alcohol, xylene, water, and the like may be used in process steps such as cleaning, rinsing, etc. Although the liquids are ultrapure, they contain traces of impurities or contaminants. Contaminants in the liquids produce a residue on the semiconductor wafers after drying. The microminiature devices on the semiconductor wafers may be rendered defective by even a slight residue from the liquids used. Thus, it is desirable to monitor impurity levels in the liquids used. Impurity concentrations on the order of tens of parts per billion may be sufficient to cause defects in semiconductor wafers. The measurement of such low impurity concentrations has been extremely difficult.
A technique for measurement of sub parts per million concentrations of impurities in liquids is disclosed in U.S. Pat. No. 4,794,086, issued Dec. 27, 1988 to Kasper et al. The disclosed technique involves the dispersion of the liquid into droplets in a gas stream The droplets evaporate in the gas stream leaving residue particles having diameters that depend on the impurity level. The sizes of the particles are analyzed using a light scattering particle spectrometer, a condensation nuclei counter or an aerodynamic particle sizer. The concentration by volume of the impurities is calculated from the droplet diameter and the particle diameter. Aerodynamic particle sizing systems are disclosed in U.S. Pat. No. 3,854,321 issued Dec. 17, 1974 to Dahneke; U.S. Pat. No. 4,895,034 issued Jan. 23, 1990 to Poole; U.S. Pat. No. 4,917,494 issued Apr. 17, 1990 to Poole et al; and U.S. Pat. No. 4,938,592 issued Jul. 3, 1990 to Poole et al. A technique for measuring impurity concentrations in a liquid employing a condensation nuclei counter is disclosed in U.S. Pat. No. 4,761,074, issued Aug. 2, 1988 to Kohsaka et al.
The prior art techniques for monitoring impurities in ultrapure liquids have had one or more disadvantages which have resulted in poor accuracy and poor resolution. In particular, the non-volatile residue technique described above requires highly accurate measurement of both droplets and particles. Such measurement techniques have not been known in the prior art, since techniques for controlling droplet generation have not been developed. Furthermore, prior art systems for monitoring impurities in ultrapure liquids have not been suitable for use on a continuous basis in a production environment.
It is a general object of the present invention to provide improved methods and apparatus for monitoring impurities in liquids.
It is another object of the present invention to provide methods and apparatus for monitoring impurity levels in liquids with high accuracy and high resolution.
It is yet another object of the present invention to provide methods and apparatus for monitoring impurities in liquids by analyzing the non-volatile residue of single droplets of the liquid.
It is a further object of the present invention to provide a non-volatile residue system having accurate control over droplet generation.
It is still another object of the present invention to provide methods and apparatus for monitoring impurities in a liquid on a continuous basis in a production environment.
It is a further object of the present invention to provide methods and apparatus for monitoring impurity concentration and impurity composition in a liquid.